1. Field of the Invention
The present invention relates to integrated circuit design, and more specifically to a method and apparatus for reducing noise in a switched capacitor amplifier circuit sampling a reference voltage.
2. Related Art
Amplifier circuits (amplifiers) are often employed to amplify signals. In general, an amplifier amplifies an input signal and provides the amplified signal as an output signal. A switched capacitor amplifier is a type of amplifier circuit, which contains operational amplifier(s) (op-amp), switches and capacitors driven by clock signals. The switches are operated to control the connections and attain the desired amplification, as is well known in the relevant arts.
Switched capacitor amplifiers are used in various environments, and an analog to digital converter (ADC) represents an example apparatus in which switched capacitor amplifiers are implemented. An example ADC implementing a switched capacitor amplifier is described below with reference to FIG. 1.
FIG. 1 is a block diagram of an example pipeline ADC implementing a switched capacitor amplifier according to a prior approach. ADC 100 is shown containing sample and hold amplifier (SHA) 110, stages 120-1 through 120-S and digital error correction block 130. Each block is described below in further detail.
SHA 110 samples an input signal received on path 134 and holds the voltage level of the sample for further processing. Each stage 120-1 through 120-S generates a sub-code corresponding to a voltage level of an analog signal received as an input, and an amplified residue signal as an input to a (any) next stage. Digital error correction block 130 receives sub-codes from the various stages, and generates a digital code (on path 146) corresponding to the sample received on path 134.
FIG. 2 further illustrates the logical operation of each stage (described with reference to stage 120-1 only, for conciseness) according to a prior approach. Stage 120-1 is shown containing flash ADC 250, digital to analog converter (DAC) 260, subtractor 270 and gain amplifier 280. Flash ADC 250 (an example of a quantizer) converts a sample of an analog signal received on path 111 into a corresponding P-bit sub-code provided on path 256. DAC 260 converts the sub-code received on path 256 into corresponding analog signal (Vdac) on path 267.
Subtractor 270 generates a residue signal as the difference of sample 111 (Vi) and the analog signal received on path 267 (Vdac). Gain amplifier 280 amplifies the residue signal (Vi−Vdac), which is provided on path 112 as an amplified residue signal. The signal on path 112 is used to resolve the remaining bits in the N-bit digital code by the subsequent ADC stages.
The subtractor, DAC, and gain amplifier may be implemented using a capacitor network and an operational amplifier, together operating as a switched capacitor amplifier. One prior switched capacitor amplifier implementing subtractor 270, DAC 260, and gain amplifier 280 of stage 120-1 is described below with respect to FIGS. 3A-3D.
FIG. 3A is a circuit diagram of a prior switched capacitor amplifier implementing subtractor 270, DAC 260, and gain amplifier 280 of stage 120-1. Switched capacitor amplifier 350 is shown containing op-amp 355, capacitors 321-324 and 331-334, and switches 311-315 and 341-345. The operation of the components is described below in further detail.
The circuit in FIG. 3A operates in two phases according to clock signals 370 and 390 shown in FIG. 3D. Stage 350 operates to sample the signals received on inputs 310 and 340 in phase one (sample phase between time points 371 and 372), and amplify and hold the sampled signal in phase two (hold phase between time points 391 and 392). The desired operations in the two phases are obtained by operating various switches, as described below with reference to FIGS. 3B and 3C respectively.
FIG. 3B is a circuit diagram depicting the operation of phase 1 in which switches 311, 313, 314, 341, 343 and 344 are closed (making contact) and switches 312, 31 5, 342 and 345 are kept open. For clarity and conciseness, the portions of the circuit not connected due to the open switches and not necessary for an understanding of the circuit operation, are not shown.
Capacitors 321 and 331 are charged to store a charge proportionate to the differential input signal received on paths 310 and 340 (due to the closing of switches 311 and 341). Capacitors 323 and 333 discharge the charges accumulated (e.g., from a previous phase operation) due to closing of switches 313, 314, 343, and 344.
FIG. 3C is a circuit diagram depicting operation of phase 2 (of the circuit diagram of FIG. 3A) in which switches 312, 315, 342 and 345 are closed (making contact) and switches 311, 313, 314, 341, 343 and 344 are kept open. One of the terminals of capacitors 321 and 331 gets connected to the reference input REFP and REFM, respectively, due to operation of switches 312 and 342.
Capacitors 321 and 331 discharge (pump) a charge proportionate to the difference of the sampled voltage and the reference voltage (residue signal) into capacitors 323 and 333, respectively. Due to the operation of switches 315 and 345, voltage developed across capacitors 323 and 333 appears at the output terminals 360 and 390 respectively. The ratio of the capacitance values of the feedback capacitor and the sampling capacitor determines the amplification factor (gain) of the residue signal (from the principle Q=CV). The amplified residue signal is provided to the next stage (120-2) on path 112.
One recognized disadvantage with approach described above is that the common mode voltage (well known in relevant art) at the input of op-amp 355 may potentially be different during phase 1 and phase 2, if sources (not shown) of input (310, 340) and reference voltages (REFP and REFM) connected to the capacitors (321, 331) have different common mode voltages. As a result, the operating point (bias point) of the operational amplifier may drift from phase 1 operation to phase 2 operation, resulting in an error or non-linearity in the digital bits generated.
In one prior approach, such an error/non-linearity is reduced by ensuring the common mode voltage of the reference signal (between time point 391-392) equals the common mode voltage of the input signal sampled between corresponding time points (371-372). However, such equality of common mode voltages may not be obtained under various operating conditions. For example, the common mode voltage of the input signal may dynamically vary on the time scale, based on the nature of the input signal being sampled.
Accordingly, the input signal and the reference voltages may be sampled together to reduce the error/non-linearity due to the difference in common mode voltage. One prior embodiment sampling both input signal and the reference voltages is described below with reference to FIGS. 4A-4C.
FIG. 4A is a circuit diagram of a prior switched capacitor amplifier, which samples an input signal and a reference signal together. Circuit 400 is shown containing capacitors 421-424, 431-434, 461 and 462, switches 411-416, 441-445, 471, 472 and 475, and op-amp 450. The operation of the components is described below in further detail.
Circuit 400 operates in two phases according to clock signals 370 and 390 shown in FIG. 3D. Broadly, in phase 1 (between time points 371-372), capacitors 421 and 431 sample an input signal and capacitors 461 and 462 sample a reference signal. In phase 2 (between time points 391-392), the difference of sampled input voltage and reference voltage (residue signal) is amplified. The desired operations in the two phases are obtained by operating various switches, as described below with reference to FIGS. 4B and 4C respectively.
FIG. 4B is a circuit diagram depicting the operation of circuit 400 in phase 1 (duration 371-372) in which switches 411, 471, 413, 414, 441, 472, 443 and 444 are closed (making contact) and switches 412, 415, 442, 445, 416 and 475 are kept open. As a result, each of capacitors 421 and 431 is connected between the input signal and a common mode reference CM, each of capacitors 461 and 462 is connected between the reference voltage and common mode voltage CM, and both terminals of each of capacitors 423 and 433 are connected to common mode reference CM as shown.
Due to such connections, each of capacitors 421 and 431 stores a charge proportionate to the sampled differential input signal (as desired and noted above), each of capacitors 461 and 462 stores a charge proportionate to the reference voltage (REFP and REFM), and each of capacitors 423 and 433 discharges any (residue) charge accumulated in the previous phase of operation.
FIG. 4C is a circuit diagram depicting the operation of circuit 400 in phase 2 (391-392) in which switches 412, 415, 442, 445, 416 and 475 are closed (making contact) and switches 411, 471, 413, 414, 441, 472, 443 and 444 are kept open. As a result, charges stored in the capacitors 421, 461, 431 and 462 are transferred to the feedback capacitor 423, 433.
The output voltage generated at the output of op-amp 450 is given by:
                    Vout        =                              Voutp            -            Voutm                    =                                                                      Cs                  ⁢                                                                          ⁢                  1                                Cf                            ×                              (                                  Vinp                  -                  Vinm                                )                                      -                                                            Cs                  ⁢                                                                          ⁢                  2                                Cf                            ⁢                              (                                  REFP                  -                  REFM                                )                                                                        Equation        ⁢                                  ⁢                  (          1          )                    wherein Cs1 represents the capacitance value of each capacitor 421 and 431, Cs2 represents the capacitance value of each capacitor 461 and 462, and Cf represents the capacitance value of each capacitor 423 and 433. Vout in Equation 1 represents an amplified residue signal provided to stage 120-2 on path 112 at time point 392.
As may be appreciated from the above, the common mode voltage at the input of the amplifier remains the same in both phase 1 and phase 2 since capacitor pairs 461/462 and 421/431 maintain the same common mode at their respective inputs. This, in turn, is because capacitors 421 and 431 sample input during phase1 and are shorted during phase2. The same applies for capacitors 461 and 462 also. In contrast, in the embodiments of FIGS. 3A-3C, the input signal voltages were maintained in phase 1 and the reference signal was connected in phase 2.
Continuing with reference to FIGS. 4A-4C, since the common mode voltage at the input terminals of the op-amp is maintained equal in both phase 1 and phase 2, the error/non-linearity is reduced in circuit 400 as against circuit 350.
However, due to addition of capacitors 461 and 462, additional noise is introduced by a stage implementing the switched capacitor amplifier of circuit 400. Accordingly, the noise power generated by the circuit of FIG. 4A, as measured at the input of circuit 400 is at least approximately given by:
                              N          p1                =                                            2              ⁢              KT                                      C              s1                                +                                                    2                ⁢                KT                                            C                F                                      ×                                          (                                                      C                    F                                                        C                    S                                                  )                            2                                +                                                    2                ⁢                KT                                            C                S2                                      ×                                          (                                                      C                    S2                                                        C                    S1                                                  )                            2                                +                                    4              3                        ×                          (                              1                +                α                            )                        ×                          KT                              C                L                                      ×                          1              β                        ×                          1                              G                2                                              +                                                    (                                  VN                  REF                                )                            2                        ×                                          (                                                      C                    S2                                                        C                    S1                                                  )                            2                                                          Equation        ⁢                                  ⁢                  (          2          )                    wherein a represents a ratio of transconductance of all noise contributing transistors in the amplifier (not shown in the figures) to transconductance of input transistors of the amplifier, CL represents the total load at the output of the amplifier, VNref represents the noise in the reference voltage, G represents a loop gain of the amplifier (equal to a ratio of the capacitance values of feedback capacitor 421 to sampling capacitor 431), K represents a Boltzmann constant, T represents ambient temperature, (as is well known in the relevant arts) and β represents a feedback factor of op-amp and equal of capacitance value of feedback capacitor to the sum of the capacitance values of all the capacitors connected to input terminals of the op-amp, given by:
                    β        =                              C            F                                              C              S1                        +                          C              S2                        +                          C              F                        +                          C              PIN                                                          Equation        ⁢                                  ⁢                  (          3          )                    
wherein CPIN represents a total parasitic capacitor at the input of op-amp.
From Equation (2), above, the noise due to capacitors 461 and 462 can be reduced by reducing CS2 and/or by increasing β. However, for reducing Cs2 without affecting the transfer function (Equation 1), the reference voltage VREF=REFP−REFM will have to be increased, which may not be possible due to supply voltage limitations.
Hence what is needed is a method and apparatus for reducing noise in a switched capacitor amplifier circuit sampling a reference voltage.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.